Pressure vessels often require access to their interiors for inspection or other servicing. Access openings must be provided in these vessels to accomplish this. Any closure device that seals these openings must be relatively easy to open and close, and must be able to withstand the internal pressure of the vessel with no leakage of liquid or gas.
Prior methods of meeting these two requirements have been time-consuming, and have required considerable use of equipment and manpower to obtain access to the inside of the vessel. The use of a closure based on U.S. Pat. No. 2,690,275, by Alt et al, is an example of this problem. After twenty-four 17/8" diameter.times.24" long bolts are first removed, a "holding" ring is removed using an overhead crane. Forty-six 13/4" diameter.times.193/4" long bolts are then removed from a sixteen piece segmented "shear" ring. The overhead crane is then used to remove the sixteen pieces of the shear ring, a "backup" ring, a "seal" ring, and a cover. This procedure requires special handling fixtures, the removal of seventy bolts, and the storage and identification of a number of parts.
Another disadvantage of closures like the one referred to above is that deflection of the pressure shell under hydrostatic pressure allows the sealing surfaces to be underloaded. The bolts of the shear ring must therefore be retorqued after pressure is applied.